This invention relates generally to an ionographic or electrophotographic printing machine, and more particularly concerns using a magnetic roll to apply and remove toner from the donor roll to achieve the desired charge of the donor roll.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed by bringing a developer material into contact therewith. Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and photoconductive member. The toner is developed on the latent image recorded on the photoconductive member by a combination of mechanical and/or electrical forces. Two component development systems have been used extensively in many different types of printing machines. A two component development system usually employs a magnetic brush developer roller for transporting carrier having toner adhering triboelectrically thereto. Two component development systems and single component development systems each have their own advantages. Accordingly, it is desirable to combine these systems to form a hybrid development system having the desirable features of each system. However, in combining these two systems, when a magnetic brush roll and a donor roll are used, proper charging of the donor roll with a toner layer is needed.
An apparatus for developing an electrostatic latent image in which developer material is transferred from a chamber to the donor roll for developing an electrostatic latent image is disclosed in a U.S. Pat. No. 5,063,875, issued Nov. 12, 1991 to J. J. Folkins. The present invention represents a development in the above-cited technology and accordingly this reference is incorporated by reference in the present specification.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 3,929,028 to Liebman describes a developer sump located below a donor roll. A developer mix of toner particles and ferromagnetic carrier granules is in the sump. A cylinder having a magnet disposed therein rotates through the developer mix and conveys the developer mix adjacent the donor roll. An electrical field between the cylinder and donor roll loads the donor roll with toner particles.
U.S. Pat. No. 4,868,600 to Hays et al. describes a scavengeless development system in which a donor roll has toner deposited thereon. A pair of electrode wires are closely spaced to the donor roll in the gap between the donor roll and the photoconductive member. An AC voltage is applied to the electrode wires to detach toner from the donor roll and form a toner powder cloud in the gap. Toner from the toner powder cloud is attracted to the latent image recorded on the photoconductive member to develop the latent image recorded thereon. A conventional magnetic brush used with two component developer could be used for depositing the toner layer onto the donor roll.